The high efficiency potential of the Stirling cycle engine has excited considerable interest since the cost and the availability of petroleum fuels has become a more widely recognized problem. The Stirling engine is an attractive approach to this problem because it has the potential for operating more efficiently and reliably than any other heat engine known today, and it is externally heated so it can be powered by virtually any heat source.
Development of the Stirling engine has uncovered some serious problems. Some of these problems are so serious and seemingly insoluable that Stirling engine development work has been abandoned altogether by certain resourceful and innovative organizations. Two persistent problems encountered by Stirling developers are engine power control and the reciprocating-to-rotating motion conversion mechanisms.
Of the control systems which have been tried heretofore, perhaps the most successful is the working gas mean pressure control (MPC) which adjusts the power level of the Stirling engine by changing the pressure of the working gas charge in the engine. The MPC system uses a high pressure working gas storage vessel which provides a gas charging source to the engine, and a gas compressor driven by the engine for compressing working gas in the storage vessel. The suction inlet of the compressor provides a low pressure dump for reduction of the engine gas charge, and a valve between the engine and the pressure vessel can be opened to increase the working gas pressure.
The MPC technique is effective to control engine power, but certain difficulties have been encountered in its practical application. For example, it was found that a large negative torque can be momentarily generated at the start of charging, which is highly objectionable. To overcome this problem, the working gas is admitted into the engine working space at certain portions of the Stirling cycle, thereby removing the negative torque difficulty. However, the timing and valving mechanism required to achieve this precisely timed admission of working gas into the working space have increased the complexity and cost of the control mechanism and decreased its reliability.
Another control technique which has been considered is the dead volume control. Engine power can be lowered by selectively connecting the engine working space to empty vessels of various volumes to increase the dead volume, that is, the volume in the engine working space not swept by the piston or the displacer. This technique does indeed control the engine power, however, it also reduces engine efficiency and changes engine power in discrete torque steps. In addition, the bulk of the resulting engine is increased and the cost increased proportionately.
Phase control is an effective power control technique for displacer engines, however the Siemens or double acting engine has a power piston which also functions as the displacer, so the piston phase is fixed and cannot be varied.
Stroke control has been used on free-piston Stirling engines to control power output. Indeed, stroke control in these engines is a naturally occurring condition wherein load resistance to the piston motion decreases the piston stroke, but increases the force exerted by the piston. Another control technique using stroke variation is in Stirling engines having a displacer and a piston. By controlling the displacer stroke, the mass flow of gas in and out of the hot and cold space can be reduced so that the range of cyclic pressure amplitude is reduced. However, it is likely that the power control achieved by this technique is a function of the displacer to piston swept volume ratio.
The other serious problem encountered by Stirling engine developers has been the reciprocating-to-rotating motion conversion mechanism. The three mechanisms most intensively addressed by workers in the art have been the crankshaft and connecting rod mechanism most commonly used in internal combustion engines, the rhombic drive mechanism, and the swashplate and the wobble plate drive mechanism. These devices are all heavy, bulky, and expensive. The latter two mechanisms require special, custom built bearings with asymmetrical loads and plane bearing surface which are expensive and require high idle speeds to maintain their bearing capacity. In addition, the crank and swashplate or wobble plate mechanism exert a side load on the piston rod which exacerbates the difficulty of sealing the working gas within a working space of the engine. Attempts to decrease the cost, bulk and side loads exerted by these mechanisms have caused increases in the noise, inefficiency, and unreliability of the mechanisms.
Accordingly, a major advance towards the introduction of a commercial Stirling engine would be made by the provision of a simple and reliable mechanism which provided both power control and reciprocating-to-rotating motion conversion in a small, quiet, and light-weight mechanism that was efficient and reliable.